1. Field of the Invention
The present invention relates to an optical glass, a glass preform made of it, an optical product and a process for the production of the optical product. More specifically, it relates to an optical glass which has high-refractivity and low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time, a glass preform which is made of the above optical glass and suitable for the formation of various optical products, an optical product made of the above optical glass, and a process for the production of the above optical product.
2. Explanation of Related Art
Conventionally, an optical glass having high-refractivity and low-dispersion optical properties contains a large amount of La2O3, Gd2O3, Y2O3, Ta2O5, ZrO2, etc., for attaining a high refractive index and a low dispersion property as is shown, for example, in JP-A-54-90218 and JP-B-54-6042, and glass-network-forming components such as B2O3 and SiO2 are contained in a small amount, so that the above optical glass is extremely highly liable to undergo crystallization. Since a glass that can be stably produced is limited in compositional range, commercially available optical glasses having a high refractive index and a low dispersion have a glass transition point Tg of over 720° C. Table 1 shows properties of high-refractivity low-dispersion optical glasses shown in brochures supplied by optical glass manufacturers.
TABLE 1RefractiveGlass transitionindexAbbe's numberpoint[nd][νd][Tg] (° C.)A1.8830040.8730B1.8806741.01758C1.8830040.8738
Conventional high-refractivity low-dispersion glasses have a very high temperature for viscous flows as is typically shown by glass transition points Tg, and for example, annealing treatment thereof has required a temperature of 710° C. or higher. Generally, most of furnaces for glass annealing are made of a stainless steel plate, and this material has a deformation temperature around 700° C. When the annealing is carried out at a temperature over 710° C., therefore, there is caused a problem that the above stainless steel plate undergoes deformation, so that it is difficult to operate the furnace for a long period of time.
Further, the production of a lens material by re-heat pressing also requires a very high temperature, which causes a heat-treating furnace to deteriorate sooner and hinders stable production.
Meanwhile, when a glass has a glass transition point Tg of 700° C. or lower, stable production has been achieved without causing any special load on the operation of facilities.